It is known to provide a plate type heat exchanger for effecting heat transfer between a first fluid, for example an oil which is to be cooled, and a second fluid, for example a coolant liquid, the heat exchanger comprising:
a casing which is provided with an inlet tube and an outlet tube for the second fluid; PA0 a stack of plate elements (which will be referred to in this specification as "half-plates"), which are disposed in pairs in opposed or "back-to-back" relationship within the casing and which define a direction of stacking, in such a way in that the half-plates of any one pair form a hollow plate of the heat exchanger defining between the half-plates of the pair a flow passage for the first fluid, with adjacent hollow plates or pairs of half-plates defining between the pairs at their peripheries, within the casing, flow passages for the second fluid which communicate with each other; and PA0 communication means adapted to allow the first fluid to flow between the successive pairs of half-plates from a first fluid inlet to a first fluid outlet.
In this known type of heat exchanger, the half-plates are identical, each being in the general form of a disc having a circular peripheral lip and a circular internal lip. Thus, when the corresponding lips of two half-plates, arranged facing each other and forming one pair, are joined together (for example by brazing), so that the two half-plates then have the flow passage for flow of the first fluid, for example oil, through the hollow plate as mentioned above, this flow passage is annular.
In order to ensure the flow of the oil from one pair of half-plates to another, each oil passage is provided with two diametrically opposed flow ports, namely an inlet port and an outlet port, each of which is bounded by a lip which is arranged to be joined sealingly to a similar lip of an adjacent half-plate.
Such a known heat exchanger is used most typically for cooling lubricating oil from an engine block, and it includes a central tube around which the disc-shaped half-plates are stacked. A threaded bar, engaged within the tube, serves firstly to secure the heat exchanger onto the engine block, and secondly to secure an oil filter onto the heat exchanger itself. This hollow tube also provides a path for the return of the oil to the engine block, either directly through the tube itself or through the threaded bar, which will then be made hollow for this purpose.
In addition, this known type of heat exchanger includes a bypass which is provided with a flap valve that is normally open when the oil is cold and viscous, and closed when the oil is hot and fluid. In the open position of the flap valve, the oil passes directly through the heat exchanger from the oil inlet to the bypass, flowing through the inlet openings of the half-plates so that it reaches the filter directly and then returns to the engine through either the central tube or the central, hollow, threaded fastening bar. Under these circumstances, the oil is not cooled. However, when the flap valve is closed, the oil is distributed into each of the oil passages defined between the co-operating half-plates, through the inlet openings of the half-plates. The oil leaves each oil passage through the outlet openings of the half-plates, to go towards a passage communicating with the filter, from which it then returns to the engine as described above. This oil is cooled by heat transfer with the coolant liquid in the heat exchanger.
It has however been established that such a heat exchanger is not well adapted to give optimum heat transfer performance.
It is also known from the French published patent application No. FR 2 214 873A, to provide a plate type heat exchanger which does enable heat transfer performance to be improved, but at the expense of a far more complex structure than that of the known type of heat exchanger described above.